Continuous inkjet printers are well known in the field of coding and industrial marking of various products, for example to mark barcodes or the expiration date on food products directly on the production chain and at high speed rate. This type of printer is also found in certain decorative fields where the graphic printing possibilities of the technology are exploited.
It is traditionally distinguished two categories within continuous inkjet printers:                on one hand, multi-deflection continuous jet printers where each drop of a single jet (or few jets) can be sent on various paths corresponding to controls for different deflections of the drops, thereby achieving a raster stroke that prints a column of dots on the zone to be printed, in a direction which is the deflection direction;        on the other hand, binary continuous jet printers where a plurality of jets placed side by side each have only one path designed for printing; the synchronous control, at a given moment, of all of the jets makes it possible to print on the medium according to a pattern corresponding in general to that of the nozzles on the nozzle plate.        
In both types of printers, the printing of a surface is achieved by the relative movement between the printing head and the medium to be printed.
As illustrated in FIG. 1, these printers include a printing head 1, generally distant from the body of the printer; it is connected thereto by an umbilical 19 bringing the hydraulic and electrical connections necessary for the operation of the head.
The head 1 has a drop generator 2 supplied with pressurized electrically conductive ink and capable of emitting one or several continuous jets 9 through nozzles, the jets being transformed into a succession of drops under the action of a periodic stimulation system situated upstream from the nozzle(s). When the drops are not intended for printing, they are directed toward a gutter 3 which recovers them in order to be recycled. Devices 4 placed along the jet (charge and deflection electrodes) make it possible, upon command, to electrically charge and deflect the drops; these drops are deviated from their natural ejection trajectory from the drop generator. The drops intended for printing escape the gutter and are deposited on the medium to be printed (not shown).
Inkjet printers also comprise a fluid circuit which performs the two basic functions, i.e. providing ink to the drop generator at a suitable pressure and with a suitable quality, and recovering, by suction, the ink not used for printing from the jets.
Inkjet printers also comprise a controller capable of managing the action sequencings (sensor output measurements, active component controls . . . ) and performing the processing enabling the activation of the different functions.
These printers lastly comprise an interface which gives the operator a means to run the printer and in return to be informed of the operation thereof.
The general opinion is that the reliable operation of an inkjet printer requires the completion of periodic maintenance interventions.
Some are manual, such as resupplying the printer with consumables (ink and solvent) to replace the consumed fluids: it is then useful, or even imperative for the printer to notify the user of the exhaustion of reserves. In this category we can also cite the changing of life-limited components or wearing parts, such as the filters or mobile pump elements through preventive maintenance. Other maintenance operations have every interest in being automatic for reasons of frequency, accessibility of the components and reliability (by repetitiveness) of execution.
The operating functions of the head are in this last category. These functions concern the jet stops and starts, the cleaning or rinsing of the drop generator, the nozzle and the gutter, and the stability checking of the jet; they contribute greatly to the overall reliability of the printer. This is why many existing printing heads are provided with hydraulic switching elements (solenoid or one-way valves) making it possible to connect the drop generator to the pressurized ink source and to a solvent source, as well as to a vacuum source. Likewise, the ink recovery gutter can be provided with a closing element, and potentially with a supply of solvent. The command sequencing for these hydraulic elements makes it possible to perform jet stops and starts optimally.
This type of arrangement is for example described in patent applications JP2001071532 by Keyence and FR 2879961A1 by the applicant.
The functions performed by a continuous inkjet printer fluid circuit according to the prior art can be broken down into two categories:                the functions, which can be called “basic”, of which there are two, which consist of providing ink at a regulated pressure to the drop generator of the head and recovering, by suction, the fluids not used for printing returning from the head,        the functions, which can be called “utility” functions, which are related mainly to the supply of consumables (ink and solvent), monitoring and control of the ink quality, maintenance of the head.        
These two types of functions have very different purposes and technical requirements. They are activated and sequenced by the controller of the printer.
Basic Functions of the Fluid Circuit:
In the prior art, we find different ways of performing the basic functions of a fluid circuit for continuous inkjet printer. The pressurization of the ink is generally done either by using pumps, which can be of various technologies, or by pressurization of a tank using compressed air in which the ink has been transferred. The vacuum or suction is generally generated either by the direct use of a pump or a hydro-ejector powered by a flow of pressurized ink, or by a tank depressurized using, for example, a venturi supplied with compressed air.
Among all of the solutions available in the prior art, there is one particularly simple, reliable and proven solution: the ink is pressurized using a gear pump (proven technology used by a large number of manufacturers of this type of printer) for example driven by a motor (direct current or step-by-step) whereof the controller can control the speed of rotation. The ink thus pressurized passes through an anti-pulsation system making it possible to damp the pressure undulations generated by the gears. This solution is for example used in the printers marketed in the name of the company Markem-Imaje under the product name 9040. The pressure of the ink is measured using a pressure sensor before being filtered by the main filter and directed toward the printing head. The pressure value measured by said sensor can be used by the controller to control the ink pressure at a given set point by acting on the speed of rotation of the motor. A second control mode is generally implemented when the jet speed is available (measured at the head), the controller can then act on the speed of the pump motor to control the speed of the jet at a given value, the pressure sensor is then used as indicator for monitoring of the machine. In general, the temperature of the ink is also measured at the outlet of the pump in order to take it into account in the different control functions of the printer.
Since the gear pumps generally have a flow much greater than the flow required for the ejection of an ink jet, it has already been proposed in the prior art, for example in U.S. Pat. No. 4,827,278 by Domino Printing Science PLC, to use this driving power in a hydro-ejector (venturi) in order to perform the second basic function, i.e. the suction necessary for the return of fluids coming from the head.
Utility Functions of a Fluid Circuit:
In order to supply the printing head with ink, most ink circuits for continuous jet printers of the prior art use at least one tank which can be described as intermediate.
Indeed, in this intermediate tank the ink of suitable quality, i.e. ink with a suitable viscosity and/or concentration, is prepared, and then supplied under pressure to the printing head. Moreover, the fluids (related ink and solvent) not used for printing returning from the head are recovered in this intermediate tank. The ink used for printing must be replaced in the intermediate tank from, in general, an external reserve (cartridge or can) provided by the user (operator) of the printer.
As previously stated, the pressurized ink which supplies the head for printing purposes must be of suitable quality. Indeed, because the solvent evaporates during the recycling of the unprinted ink, the viscosity and/or concentration of the ink must generally be adjusted periodically by adding solvent in the intermediate tank, in general from an external reserve (cartridge or can) of solvent provided by the user of the printer.
Thus, a first utility function consists of determining the quantity of ink. In the prior art, this involves detecting characteristic levels of ink in this intermediate tank. Given the expected characteristics of the fluid circuit in general, it is only necessary to detect two or three discrete levels in this intermediate tank: a high level to make it possible to avoid overflowing, a working level which the controller will try to maintain by adding new ink, and a low level to make it possible to avoid ingestion of air by the ink pressurization system. In certain cases, only the high and low levels are exploited.
Many discrete level detector technologies have been used in the prior art, one of the most reliable and easiest to implement uses the principle of rod level sensors dipping into the tank; this principle takes into account the fact that the liquid to be detected is conductive. The resistivity is measured between two rod level sensors dipped into the tank, and if the ink short circuits the rods, the drop in resistivity is detected to declare a presence of ink at that level. This system remains, however, costly due to the electronic protections which the standards require be implemented when electrical currents pass in flammable environments, which is in general the case of ink with volatile solvent. Furthermore, this type of detector cannot be used with insulating fluids as solvents generally are.
A second utility function is the viscosity measurement. In the prior art, the viscosity is often measured by determining the time necessary for the flow of a given quantity of fluid through a calibrated hydraulic restrictor. This device generally requires the implementation of dedicated means: a measuring cavity, at least two level detectors, hydraulic switching means to fill and empty the cavity.
Quasi-identical means are necessary to implement a rolling ball viscosity meter which is also found in the prior art (for example as shown in application WO 2007/129110). In this type of viscosity meter, the lowering speed of a ball in a vertical tube having an internal diameter slightly larger than the diameter of the ball is representative of the viscosity of the fluid contained in the tube. These devices require the implementation of a significant number of components. The evaluation of the viscosity can also be done without viscosity meter, in a continuous inkjet printer, by measuring the parameters of the jet when it is operational, and its speed, when possible. Indeed, one can identify, for a given situation (ink and nozzle in particular), a characteristic connecting the viscosity of the ink passing through the nozzle to the speed of the jet, for a measured ink pressure upstream from the nozzle and for a measured ink temperature (patent by Company Imaje EP 0 362 101 B1). This method does not provide all of the desired flexibility in all situations, in particular due to the need to have an operational ink jet, i.e. effectively ejected by the head at a speed close to the nominal speed, to perform the measurement.
A third utility function consists of correcting the viscosity (or concentration) of the ink contained in the intermediate tank. The major drawback of the solutions used by the prior art is that the quantity of solvent making it possible to correct a viscosity gap of the volume of ink contained in the intermediate tank can only be crudely evaluated since, on one hand, the concerned volume of ink is not precisely known, and on the other hand the volume of solvent added is also not precisely known. This is due to the fact that the means used do not allow it (time for passage of a poorly-defined flow of solvent through a distribution member: solenoid valve or pump). An approximate control of the viscosity in relation to the expected viscosity is of little consequence when robust inks are used but limits the possibilities for using the printer with sensitive inks.
Other utility functions are useful in order to decrease the risks of hazardous manipulations or to increase user comfort.
For example, it is interesting to evaluate the quantity of consumable available in the replacement reserves of consumed fluids. According to the prior art, solutions can consist of transferring cans (bottles) of consumable product into auxiliary tanks integrated into the fluid circuit. These tanks are provided with a level detector (Series S8 printer marketed by the company Imaje).
One can also use sealed and removable consumable cartridges which are tightly connected to the fluid circuit as needed. In this case, the evaluation of the quantity of consumable remaining in the cartridges is done using means external to the cartridges themselves, possibly requiring the implementation of dedicated sensors as described in patent application WO2009047497 by the company Videojet. The solution according to this document consists of considering that the quantity of remaining fluid is connected by a characteristic to the vacuum created by the withdrawal of the fluid from a semi-rigid sealed cartridge. This solution requires the implementation of a dedicated pressure sensor.
In other words, the implementation of these utility functions requires the use of many components with their control (electronic) members.
By inventorying commercial solutions and solutions described in the literature, the inventors came to the conclusion that there are, to date, three categories of design solutions for performing the basic functions and, if applicable, utility functions, of continuous jet printer ink circuits:
1/ a category according to which most of the functions of a fluid circuit are implemented independently using distinct means dedicated to each function. This solution, very often adopted by the suppliers of continuous inkjet printers, has advantages: on one hand, the components can be perfectly dimensioned for the concerned function and therefore be technically high-performing, and on the other hand the interactions between functions are reduced, which makes the operation of the fluid circuit robust and easier to develop. However, the number of components and associated control interfaces, the difficulty of assembly and the resulting bulkiness of the system lead to prohibitive production costs and a non-optimal commercial situation.
2/ a category using the elements of the preceding category but with a decreased number of components, to the detriment of the performance of the printer or the service provided to the user. These machines are intended for highly cost-sensitive markets which tolerate the induced limitations. These printers cannot be proposed for demanding applications. One solution in this category is illustrated in patent application WO2007/129110 in the name of the company Domino: it consists of using the removable renewal tank as intermediate tank and consumable reserve. Moreover, the levels in the tanks are not measured using detectors, but the remaining quantities are evaluated from the knowledge of the initial volumes present in the renewal tanks at the time of the change and an estimate of the ink and solvent consumption. The major drawback is that the evaluation is approximate, which makes it necessary to signal empty tanks (to be changed) with a sufficient safety margin, in order to avoid the ingestion of air by the head, well before the tanks are completely empty. This results either in losing a large quantity of consumable, or requiring the user to visually monitor the level of the tanks, which is not practical. Moreover, the absence of an intermediate tank leads to stopping printing during the changing of the removable tanks in order to avoid ingesting air, which would lead to triggering time-consuming maintenance operations.
3/ the third category can be analyzed as design solutions which get around the drawbacks of those of the first category without making compromises on the essential needs of a good-level printer. Thus, here it is a matter of performing both types of functions (basic and utility) of the ink circuits using shared means. This makes it possible to use fewer components and ensure greater compactness of the fluid circuit, but at the cost of significant complexity and a delicate reliability to master. Patent application WO88/04235 by the applicant describes a compact fluid circuit where many functions (utility and basic) can be performed from a variable volume cell connected to a pressure sensor and a multitude of solenoid valves making it possible to withdraw and direct the fluids into different tanks. The different functions are managed sequentially (in series); this efficient system is still, however, particularly complex to develop due to the critical aspect of the timings between the phase of the variable volume cycle and the control of the solenoid valves. This is complicated by the need to manage the response time of the different actuators of the system. The specific characteristics of the variable volume cell make it a sensitive component developed on specific needs. The large number of solenoid valves poses a reliability problem which requires technically high performances.
In the end, the drawbacks of the continuous inkjet printer ink circuits of the prior art according to their design can be summarized as follows:                ink circuits in which each function is performed independently of the other functions: they consist of an assembly of simple solutions, but use many components to be integrated and controlled, which leads to a bulky and costly assembly;        circuits with a sophisticated design to decrease the number of components (cost), but the complexity and reliability-related risk increases, by adding the development difficulty. The need to develop non-standard hydraulic components impacts the cost-effectiveness of the final product;        ink circuits with a very simplified architecture in order to obtain a low cost, but the technical and functional compromises lead to poor performance or decreased performance offered to the user and increased risk related to the feedback of insufficiently precise alarms.        
An object of the invention is therefore to overcome all or part of the aforementioned drawbacks.
One aim of the invention is therefore to simply and reliably design a fluid circuit in a continuous inkjet printer which performs the basic functions and at least the utility function of determining the quantity of ink for printing.
Another aim of the invention is to propose a mechanical sub-assembly of a fluid circuit which performs at least the basic functions and at least the utility function of determining the quantity of ink for printing, which is simple and inexpensive to manufacture.